“Low-voltage” typically refers to voltages of up to approx. 1000 Volt. If the switching isolating distances are designed accordingly, such switching devices can also be designed for switching voltages above 1000 Volt, e.g. up to 6.3 kV. In particular electrical switching devices of this type, such as low-voltage circuit-breakers, are designed to interrupt current paths in the event of an overcurrent or a short-circuit. They can be embodied as single-pole or multi-pole, in particular three-pole.
A switching mechanism for a low-voltage circuit-breaker is known from European patent application EP 0 555 158 A1. It has a movable contact, the manual opening and closing of which, as well as automatic opening because of a fault, such as a short-circuit, is controlled via the switching mechanism. The switching mechanism furthermore has a latch, articulated on a fixed spindle and having a cam surface. The switching mechanism further comprises a toggle formed by a toggle spindle, a lever linked on the one hand to the movable contact and on the other hand to the toggle spindle, as well as a lever articulated on the one hand on the latch and on the other hand on the toggle spindle. The switching mechanism further has a spring attached on the one hand to the toggle spindle and on the other hand to a handle for opening and closing the contacts.
The switching mechanism comprises a lock designed to cooperate with the latch to hold the latter in the locked position. It further comprises a cam-follower roller actuated by the handle and designed to cooperate with the cam surface of the latch to move the latch to the locked position, the handle being able to move to three distinct positions: a closed position in which the spring urges the toggle to an extension position, a manual opening and resetting position where the spring urges the toggle to a broken position wherein the latch is in the locked position, and a tripped position in which the latch is unlocked and the toggle broken. The cam surface comprises a slope change point marking the limit between two successive sections, a first section corresponding approximately to the travel of the handle between the tripped position and the reset position, and a second section corresponding to the reset position, the slope of the first section being such that the resultant of the forces derived from the spring and exerted on the handle urges the handle to the tripped position, and the slope of the second section being such that the handle is urged to the reset position arranged as a stable position holding the latch.
The disadvantage of such a switching mechanism for an electrical switching device is that can result in an undesired tripping of the switching mechanism if the latch is in the locked position and the handle is in the open position. In the case of improper transport of the electrical switching device the result can be that a tripping mechanism of the electrical switching device acts on the lock such that the latch leaves its locked position and thus trips the electrical switching device.
DE 693 06 822 D2 discloses a switching mechanism for a circuit-breaker. In this switching mechanism the handle and the latch can be held in the loaded position in the event of unwanted tripping. The switching mechanism is here characterized in that a cam surface of the latch has a slope change point marking the limit between two successive sections, a first section corresponding approximately to the travel of the handle between the tripped position and the reset position, and a second section corresponding to the reset position, the slope of the first section being such that the resultant of the forces derived from the spring and exerted on the handle urges the handle to the tripped position, and the slope of the second section being such that the handle is urged to the reset position arranged as a stable position holding the latch. Thanks to the profile of the cam surface the reset position of the handle becomes a stable position in which the handle holds the latch regardless of the position of the lock. Although a tripping action of a tripping mechanism which releases the lock cannot be prevented, this has no effect, since the latch is held by the handle. In this way any risk of the tripping and locking system being destroyed, which could occur if these systems are mechanically blocked to prevent an undesired release action, is excluded.